Okay first when you make your character make sure to have their birthday after the eighth month. This is important later on.

Then you need to build up some sin and luck. You also need to work for the shopkeeper a little bit to open up the job option with Robin the thief that appears occasionally (as a side note if you work at a waitress at the inn you can also chat with Robin early on about the recipe cards).

Once you can start working with Robin do so as often as possible. Note you want to get caught occasionally as that opens up dialogs with Robin. I won't get into the individual conversations, but eventually after getting caught enough times Robin asks you to come up with a plan for a "big heist" at the castle.

At this point you need to go to the castle during the summer festival. To play it safe I raised my mind a lot too, but I'm not sure if you need to. Once you've gone to the summer festival (I specifically went to the dance, don't know if it works with the other two options) the next time you see Robin you should be able to tell them in a chat that you know how to do it.

It requires dressing up (so make sure to have a dress bought for Robin) and going to the next summer festival, so it's important that you come up with the plan by your nineteenth birthday otherwise you won't have time.

A year passes and you do what you want and then for me both times it came around the end of the festival Robin will offer a chat option. At that point Robin will say that they're going in alone and to give them the dress. They'll then contact you later if they're successful and they suggest you go to the dance if you're nervous.

I usually do so I don't know if anything happens if you don't visit the festival. Afterwards Robin disappears. Go home to your parents on the first day of August and there will be a package waiting for you with a note from Robin saying they're at the beach.

Go to the beach and take a vacation. Note you must take a vacation, but you can redeem a coupon for it if you've won the dance and it'll work. You'll meet Robin while on vacation....